This invention relates generally to a method of making thermosetting composites and the resulting product which preferably comprises electrical circuit laminate materials. More particularly, this invention relates to an electrical circuit laminate comprising (1) a thermosetting resin of polybutadiene or polyisoprene; (2) a woven fibrous web impregnated with the resin; and (3) inorganic particulate filler such as silica, titania and the like. In accordance with an important feature of this invention, the filler loadings for both the woven web and particulate filler are selected such that uncured individual layers (prepreg) are relatively tack free thereby allowing ease of handling for lamination without the need for B-staging. The material of this invention allows for relatively low lamination temperatures.
Commonly assigned U.S. Pat. No. 5,223,568 (which is fully incorporated herein by reference) describes a thermosetting composition which is particularly useful for making electrical substrate materials. In general, U.S. Pat. No. 5,223,568 describes a composition formed from the steps of:
(a) providing a moldable thermosetting composition that includes (1) polybutadiene or polyisoprene resin which is a liquid at room temperature and which has a molecular weight less than 5,000 and (2) a solid butadiene or isoprene-containing polymer capable of cross-linking with the polybutadiene or polyisoprene resin; PA1 (b) forming the composition into a shape; and PA1 (c) curing the composition to produce the electrical substrate material including subjecting the composition to a high temperature cure condition at a temperature greater than about 250.degree. C. and less than the decomposition temperature of the composition. This composition thus comprises a two component system, the first component being the polybutadiene or polyisoprene resin and the second component being the solid butadiene or isoprene-containing polymer, all of which are subjected to the high temperature curing cycle (e.g., greater than 250.degree. C.).
In preferred embodiments, the solid polymer is a thermoplastic elastomer block copolymer.
U.S. Pat. No. 5,223,568 also describes a composition with a dielectric filler (i.e., a material having a dielectric constant greater than about 1.2 at microwave frequencies) homogeneously dispersed throughout the composition to the extent that when the composition is cured the properties of the cured article, e.g., dielectric constant and coefficient of thermal expansion, do not vary more than about 5% throughout the article.
In preferred embodiments, the composition of U.S. Pat. No. 5,223,568 further includes a crosslinking agent capable of co-curing (i.e., forming covalent bonds) with the polybutadiene or polyisoprene resin thermoplastic elastomer, or both. Examples of preferred crosslinking agents include triallylcyanurate, diallylphthlate, divinyl benzene, a multifunctional acrylate, or combinations of these agents.
When the electrical substrate material disclosed in U.S. Pat. No. 5,223,568 includes a dielectric filler, the volume % of the filler (based upon the combined volume of resin, thermoplastic elastomer, crosslinking agent, if any, and filler) is between 5 and 80%, inclusive. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres); corundum, wollastonite, polytetrafluoroethylene, aramide fibers (e.g., Kevlar), fiberglass, Ba.sub.2 Ti.sub.9 O.sub.20, glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, or magnesia. They may be used alone or in combination.
The method disclosed in U.S. Pat. No. 5,223,568 provides a wide variety of shaped articles having favorable isotropic thermal and dielectric properties. These properties can be tailored to match or complement those of ceramic materials, including gallium arsenide, alumina, and silica. Thus, the cured articles can replace ceramic materials in many electronic and microwave applications, for example, as specialized substrates for high speed digital and microwave circuits. Examples of microwave circuits include microstrip circuits, microstrip antennas, and stripline circuits. The cured products are also useful as rod antennas and chip carriers.
While well suited for its intended purposes, the circuit laminate materials described in U.S. Pat. No. 5,223,568 do suffer from several drawbacks. For example, these prior art materials require a high temperature cure (e.g., lamination) of greater than about 250.degree. C. (482.degree. F.); and this requirement is problematic for several reasons. First, conventional circuit fabrication equipment often exhibit temperature limits of 360.degree. F. (182.degree. C.), which is below that of the 482.degree. F. requirement. Still another drawback is that flammability ratings required by Underwriters Laboratory UL 94-VO lead to the need for inclusion of a flame retardant additive such as a bromine-containing fire retardant. Unfortunately, typical bromine-containing fire retardant additives cannot withstand the high temperature cure conditions of greater than 250.degree. C., undergoing decomposition and/or chemical degradation at these temperatures.
In addition to the foregoing need for lowering the cure (lamination) temperature of the circuit laminates, there is also a problem with the inherent tackiness associated with the polybutadiene or polyisoprene laminate prepregs. Because of the tacky nature of the prepreg, it is not practically feasible to use a continuous, automated layup process for laminating circuit materials. Roll lamination equipment is commonly available, however, and the ability to provide a polybutadiene or polyisoprene based prepreg of the type described in U.S. Pat. No. 5,223,568 which is also usable in an automatic layup process would greatly reduce the manufacturing cost and processing time, leading to a significant increase in the commercial success of the resultant circuit material products.
In addition to U.S. Pat. No. 5,223,568, there are other prior art patents and literature of some interest with respect to the use of filled circuit laminates in general, and polybutadiene based circuit laminates in particular. For example, in an article entitled "A New Flame Retardant 1,2-Polybutadiene Laminate" by N. Sawatari et al., IEEE Transactions on Electrical Insulation, Vol. EI-18, No. 2, April 1983, the limitations on presently-used polybutadiene based laminates is discussed, including the fact that 1,2-polybutadiene (PBD) is difficult to control in the semi-cured condition (the B-stage), difficult to make tack free, highly flammable, and exhibits low copper bond. The article thereafter describes a composition which addresses these issues. The composition described uses a high percentage of very high molecular weight PBD to eliminate tackiness and B-staging. Also, a low molecular weight, modified PBD resin is used as a minor component to aid in copper bond and flow during lamination. There is no mention of using filler of any type.
U.S. Pat. No. 5,264,065 to Kohm describes a base material for printed wiring boards where inert filler is used to control Z-axis coefficient of thermal expansion (CTE) in fiberglass-reinforced thermoset resins. The Kohm patent discloses a range of 45-65 weight % fiberglass reinforcement and a range of 30 to 100 parts filler per 100 parts of the polymer. There is no disclosure in Kohm of a polybutadiene or like material for the resin system.
U.S. Pat. No. 4,997,702 to Gazit et al. discloses a circuit laminate having an epoxy resin system which also includes inorganic fillers or fibers in the range of 20-70 weight % of the total composite. The fibers include both glass and polymeric fibers and the fillers include clay or mineral (e.g. silica) particulate fillers.
U.S. Pat. No. 4,241,132 to Pratt et al. discloses an insulating board comprising a polymeric matrix such as polybutadiene and a filler consisting of polymeric filler, for example fibrous polyproylene. In all cases, the dielectric constant or dissipation factor of the resin matrix is matched to the fibrous reinforcement in order to obtain an isotropic composite.
European Patent No. 0 202 488 A2 discloses a polybutadiene-based laminate wherein a high molecular weight, bromine-containing prepolymer is used to reduce tack and flammability of a 1,2-polybutadiene resin. Similarly, in Japanese Patent No. 04,258,658, a high molecular weight compound is added to a tacky PBD resin to control tack. The compound utilized is a halogen-containing bismaleimide which provides flammability resistance, as well as good copper bonding and heat resistance. There is no mention of the use of fillers and the resulting laminate will have a relatively high dissipation factor.
An article entitled "1,2-Polybutadienes-High Performance Resins for the Electrical Industry", by R. E. Drake, ANTEC '84 pp. 730-733 (1984), generally discloses conventional polybutadiene resins for use in laminates and specifically discloses the use of reactive monomers to co-cure with the PBD.
U.K. Patent Application No. 2 172 892 A generally discloses laminates composed of styrene-containing and thermoplastic copolymers with unsaturated double bonds and polybutadiene.
Notwithstanding the foregoing examples of laminate composites, there continues to be a need for improved polybutadiene laminates having a combination of electrical, chemical, mechanical and thermal properties which are not presently available including flame retardance, improved coefficient of thermal expansion, ability to tailor dielectric constant, improved dissipation factor, low cost and low tackiness.